1. Field of the Invention
Embodiments of the invention relate to photomasks or reticles for use in photolithography, and more particularly to reticles in which the clear feature comprises light of two different phases.
2. Description of the Related Art
Optical systems are widely used in the microelectronics industry to manufacture semiconductor wafers by a process known as, photolithography. Typically, a photolithography system comprises a light source configured to project light rays to a condenser lens. The condenser lens collimates the light rays towards a pellicle placed before (or after) a reticle, sometimes referred to as a photomask. Typically, the reticle includes an optically transparent substrate underlying an optically opaque layer in a pattern used to project an image onto a portion of a photoresist-coated wafer. The pellicle is a substantially thin, optically transparent film which seals off the reticle surface from airborne particulates and other forms of contamination. Typically, a projection lens is placed after the reticle to receive and focus the pattern of light rays onto an exposure field on a photoresist (“resist”) layer overlying a wafer. Exposed or unexposed portions of the photoresist are then developed, replicating the reticle pattern in the photoresist layer.
Improved systems and methods of photolithography have permitted formation of line features in integrated circuits below 100 nm. In some such systems, dipole illumination is used as a light source. As recounted by Mark Eurlings et al. in their paper, “0.11 μm Imaging in KrF Lithography Using Dipole Illumination,” Proc. of SPIE, Volume 4404 (April 2001) pp. 266-278, the disclosure of which is hereby incorporated in its entirety, dipole illumination advantageously enhances the exposure latitude and depth of focus for dense structures. However, the inventor has found using dipole illumination to be problematic for printing certain structures. In particular, when lines are oriented in the preferred direction of a dipole source, imperfections have been found in the interior of the line. These imperfections are called scumming, and they undesirably reduce the quality of the features formed on the integrated circuit. Features having a width substantially smaller than the space to the next feature being defined by the same mask, such as isolated trench lines, appear to be more susceptible to scumming than features printed in dense portions of the integrated circuit. For example, a dense array of lines may include parallel lines separated by a distance approximately equal to the width of the lines, while an isolated trench line may be separated from the next feature formed by the same mask by a distance greater than the width of the line, for example greater than or approximately equal to about five times the width of the trench line. Accordingly, a reticle is desired that reduces scumming and thereby enhances image quality on the integrated circuit.